In watercraft, a shaft for the propeller typically extends through one or more bulkheads separating ship compartments. If a compartment is breached, water can flow into an adjacent compartment via the gap between the bulkhead and the shaft.
Accordingly, bulkhead seals have been developed. See, for example, U.S. Pat. No. 4,403,779. In another example, Wärtsilä Propulsion UK (4 Marples Way, Havant, Hants PO9 1NX, UK) developed a “ND” and “NS” type seal including a diaphragm seal which rotates with the shaft within a housing fixed to the bulkhead. A watertight seal is supposed to be created between the diaphragm and the housing only in the presence of water when the water pressure urges the diaphragm seal into engagement with the housing.